Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4, 2003 in the name of inventors Paul Meadows et al., which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. A Spinal Cord Stimulation (SCS) system typically includes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitter and receiver, electrodes, at least one electrode lead, and, optionally, at least one electrode lead extension. The electrodes, which reside on a distal end of the electrode lead, are typically implanted along the dura of the spinal cord, and the IPG or RF transmitter generates electrical pulses that are delivered through the electrodes to the nerve fibers within the spinal column. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing to create an electrode array. Individual wires within one or more electrode leads connect with each electrode in the array. The electrode lead(s) exit the spinal column and generally attach to one or more electrode lead extensions. The electrode lead extensions, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG or RF receiver is implanted. Alternatively, the electrode lead may directly connect with the IPG or RF receiver. For examples of other SCS systems and other stimulation system, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties. Of course, implantable pulse generators are active devices requiring energy for operation, such as is provided by an implanted battery or an external power source.
An IPG may include one or more output current sources/sinks that are configured to supply/receive stimulating current to/from the electrodes on the IPG, and ultimately to/from the patient's tissue. For example, FIG. 1 shows a basic output current source 500 and a corresponding output current sink 501 used to stimulate tissue, exemplified generically as a load 505 (R). As one skilled in the art will understand, transistors M1 and M3 in the output current source 500, and transistors M2 and M4 in the output current sink 501, comprise a current mirror. The current mirrors operates to mirror a reference current, Iref, in the output stage of the current source or sink, i.e., Iout=Iref. The reference current Iref can also be scaled by providing paralleled numbers (M) of output transistors (i.e., M1 and M2), such that Iout=M*Iref. Selection of the various current sources or sinks is typically provided by selection transistors 513 and 513′. As already alluded to, an IPG typically has several electrodes, and the various current sources and sinks can be controlled to source or sink current to any particular electrode, E, as is efficacious for treating a particular patient. As shown in FIG. 1, the current source 500 is connected to IPG electrode EX while the current sink is connected to electrode EY.
The output current sources and sinks 500, 501, as one can notice from FIG. 1, are typically formed of transistors of differing polarities. Thus, the sources 500 are formed from P-channel transistors, while the sinks 501 are formed from N-channel transistors. Without a full discussion of transistor physics, one skilled will recognize that use of transistors of such polarities is sensible, given that the sources 500 are typically tied to a positive voltage (V+, referred to herein as the “compliance voltage”), while the sources 501 are typically tied to a more negative voltage, such as ground. (A “ground voltage” as used herein should be understood as any reference voltage with respect to the compliance voltage). (The substrate connection (not shown) for the transistors would typically be tied to the appropriate power supply, either V+ or ground, but could also be tied to the transistors' sources). Because the current sources and sinks 500 and 501 are generally digitally controllable as will be seen (e.g., by transistors 513, 513′), to produce output currents Iout of a desired amplitude, such current sources and sinks are typically referred to as Digital-to-Analog Converter circuitry, or “DAC” circuitry. More specifically, in reference to the polarity of the transistors in each, the current sources 500 are typically referred to as “PDACs,” while the current sinks 501 are typically referred to as “NDACs.”
Different output source/sink architectures can be used in an IPG, and are shown in FIGS. 2-4 respectively. The architecture shown in FIGS. 2A-2B is disclosed in U.S. Pat. No. 6,181,969, which is incorporated herein by reference in its entirety. As shown in FIG. 2A, in the architecture of the '969 patent, each electrode EX has its own dedicated PDAC and NDAC circuitry, which allows that electrode to either operate as a source of sink of current, or neither. As shown, the PDAC (current source) associated with electrode E2 is active, while the NDAC (current sink) associated with electrode E3 is active, thus producing the current path shown. FIG. 2B shows the PDAC circuitry for a particular electrode useable in the architecture of FIG. 2A. (Only the PDAC circuitry is shown, but one skilled in the art will recognize that the NDAC circuitry for a given electrode would be similarly formed of N-channel devices). As shown, and as one skilled will appreciate, selection transistors 513 are used to digitally set the amplitude of the current to be sourced at electrode EX (i.e., electrode E2 of FIG. 2A) from Iref to 127 Iref in increments of Iref. As this is explained in detail in the above-incorporated '969 patent, it is not further discussed.
The current architecture of FIGS. 3A-3B is disclosed in above-incorporated U.S. Pat. No. 6,516,227. This architecture is similar to that of FIGS. 2A-2B in that a number of discrete PDAC current source circuitry blocks and NDAC current sink circuitry blocks are provided. However, the PDACs and NDACs are not dedicated to any particular electrode, and instead, each PDAC and NDAC can be coupled to any given electrode via a low-impedance switching matrix, which in reality contains a number of switches to accomplish this task.
Another current sourcing and sinking architecture is disclosed in U.S. patent application Ser. No. 11/177,503, filed Jul. 8, 2005, which is also incorporated herein by reference in its entirety, which is summarized with respect to FIGS. 4A-4C. In this architecture, there is not a discrete plurality of PDAC and NDAC circuit blocks to service the various electrodes. Instead, the current source and sink circuitry is effectively distributed such that they can service any of the electrodes. Thus, a master reference current Iref (which can be scaled from another reference current I1 using a DAC 407 as shown) is used as the input to a number of scalable current mirrors 410 (FIG. 4B). Any one of the current mirrors 410 can be chosen to participate in the current produced at a particular electrode EX via a switch block 405. Thus, there is a switch block 405 associated with each current mirror 410, in which each switch block has a switch SX to allow the current from the associated current mirror to be passed to a particular electrode EX.
Regardless of the current source/sink architecture used, all generally have similar current output path characteristics. That is, and referring again to FIG. 1, the current output paths in each architecture comprises, at a minimum, a current source output transistor (or transistors if paralleled for current gain) (M1), a selection transistor to control the flow of the current mirror output transistor(s) (513), the load (R), a current sink mirror transistor or transistors (M2), and a selection transistor to control the flow of the current sink mirror transistor(s) (513′). Each of these elements has some resistance, and hence some amount of the compliance voltage, V+, will be dropped across these elements when current is flowing to stimulate the load, R. Specifically, the compliance voltage V+ will equal VDS1+VR+VDS2, where VDS1 comprises the drain-to-source voltage drop across output transistor(s) M1 and selection transistor 513, VDS2 comprises the drain-to-source voltage drop across output transistor(s) M2 and selection transistor 513′, and VR equals the voltage drop across the load.
Notice that the M1/M3 and M2/M4 current mirrors require that transistors M1 and M2 operate in a saturation mode, such that the channels of the transistors are in “pinch off.” When in saturation mode, the output current Iout is proportional to the gate voltage of the transistors M1 or M2, but does not depend upon the drain voltage to the first order. However, to keep the transistors M1 and M2 in the saturation mode, a certain drain-to-source voltage, VDS, has to be satisfied for each transistor. Specifically, VDS must be greater than the gate-to-source voltage (VGS) minus the threshold voltage (VT) of the transistor (i.e., VDS>VGS−VT). This saturation condition is necessarily satisfied because VDS=VGS by virtue of the common gate/drain connection of transistors M3 and M4. The minimum drain-to-source voltage VDS that satisfies this relationship and allows transistors M1 and M2 to operate in the saturation mode is typically on the order of a volt.
What this means in the context of the output current circuitry of FIG. 1 is that the circuit can operate properly over a range of compliance voltages, V+. For example, suppose a suitable therapy for a patient suggests that a current of Iout=5 mA should be passed between electrodes EX and EY on the IPG. Suppose further that the load R equals 800 ohms. When the current of 5 mA is passed through the load, a voltage VR=4V will build up across the load (V=I*R). Suppose further for simplicity that the minimum drain-to-source voltage to keep the output transistors M1 and M2 in saturation equals 1V when the effects of the selection transistors 513, 513′ are included. (The actual value can be different, but is chosen as 1V for ease of illustration). To provide this current, a minimum compliance voltage, V+ of at least 6V would be needed; if V+<6V, the circuitry will be unable to produce the desired amount of current.
However, the compliance voltage V+ could be higher than 6V while still producing the proper amount of current. For example, suppose for the same example that the compliance voltage V+ is 8V. In this case, the circuitry is still capable of providing the 5 mA current, and the load (which doesn't change) will still drop 4V at that current. What this means is that the remainder of the compliance voltage must be dropped across the output transistors M1 and M2 as well as their associated selection transistors 513 and 513′, e.g., 2V if the source and sink are matched.
However, running the circuit in this example with an 8V compliance voltage is not efficient. While circuit performance is the same at both 6V and 8V, i.e., both are capable of generating a 5 mA current, the former will draw only 30 mW of power (P=I*V), while the latter will draw 40 mW of power. In other words, 10 mW of power are needlessly dropped across the output transistors M1, M2 and their selection transistors 513 and 513′. This waste of power is regrettable in the context of an implantable medical device such as an IPG. As noted earlier, an IPG typically runs from a battery, and therefore it is important to minimize circuit operation that would otherwise needlessly drain the battery and cause the IPG to cease functioning, or needlessly require the patient to more frequently recharge the battery.
Unfortunately, it is difficult to design the compliance voltage to an optimal level. Depending on the electrodes stimulated, the magnitude of current required for efficient therapy for a given patient, and the resistance of the patient's flesh, an optimal compliance voltage from the vantage point of power conservation is variable.
Accordingly, the implantable stimulator art, or more specifically the IPG or SCS system art, would be benefited by techniques for sensing and adjusting the compliance voltage in a manner respectful of the power available to the device. Such solutions are provided herein.